The candidate, Dr. Ann Mullally, presents a 5-year career development plan that seeks to investigate the functional and molecular properties of myeloproliferative neoplasm (MPN) hematopoietic stem cells (HSCs), using in vivo murine models, while establishing an academic career as an independent physician-scientist in the field of hematology. The specific aims of this proposal are to (1) perform detailed functional characterization of Jak2V617F mutant HSCs; (2) investigate the effect of Tet2 loss on Jak2V617F mediated MPN and (3) determine molecular dependencies and co-operating pathways in Jak2V617F mutant HSCs. MPN are clonal disorders of hematopoiesis characterized by an accumulation of mature blood cells and development of the associated complications of this. The JAK2V617F gain-of-function mutation is the most common molecular abnormality in MPN, and importantly it arises in the HSC compartment. Therefore, it remains an attractive therapeutic target in MPN, although efforts to target it directly using small molecule JAK2 kinase inhibitors have so far failed to selectively and consistently diminish the JAK2V617F mutant clone in MPN patients. The major goal of this proposal is to gain biological understanding of the differences between normal and Jak2V617F mutant HSCs, and in doing so, to identify the therapeutic susceptibilities of MPN stem cells, so that they can be effectively targeted to definitively eliminate the disease-maintaining MPN clone. Dr. Mullally is well qualified to carry out the research outlined in this proposal, having recently described in Cancer Cell, the differential effects of the Jak2V617F mutation on hematopoietic stem and progenitor cells in a murine knockin model, that will be the main biological reagent utilized in the experiments outlined in this proposal. During the course of characterizing the Jak2V617F knockin mouse model, Dr. Mullally gained expertise in all of the technical and methodological skills required to successfully complete the objectives of this proposal. The candidate's primary career development goal during this K08 award period will be to gain the additional skills required to become an independent physician-scientist. This will be achieved through the candidate's execution of the proposed research strategy in addition to her involvement in a series of formal meetings and didactic educational activities as outlined in the career development plan. Furthermore, the candidate has assembled a highly talented team of mentors, advisors, and collaborators to shepherd her transition to independent scientific investigator. Her primary mentor, Dr. Benjamin Ebert is well positioned to provide expert guidance for this award given his background in HSC biology, erythropoiesis and RNA interference. Despite his relatively junior faculty position at Harvard, Dr. Ebert has already achieved remarkable academic success and has established a reputation for exceptional dedication to his mentees. In addition, the candidate will be co-mentored by Dr. Stuart Orkin, who has mentored more than seventy highly successful academic physicians and scientists in the field of hematology and has decades of experience in murine models of hematopoiesis. Her mentors will meet with the candidate at least monthly to supervise and assist in her transition to independence. The candidate is also fortunate to continue to benefit from the guidance and expertise of Dr. Gary Gilliland, who has an outstanding record of mentoring physician-scientists to independence, and although recently moved to industry remains very committed to the candidate's career development. Additional murine expertise will be provided by Dr. Scott Armstrong, who will serve on the candidate's advisory committee, and is recognized as a leader in the field of leukemia stem cell biology. Drs. Nancy Berliner and David Williams, both division chiefs with a long track record of mentorship, will comprise the remaining members of the candidate's advisory committee, which will meet at minimum, every six months to ensure a successful scientific research program. Finally, the candidate has recruited Dr. Ross Levine, one of the initial scientific investigators to describe the JAK2V617F mutation in MPN, as a collaborator for her project.